There's no going back now!

Well the big red and black will be for power, you could try extending them with your twin core, and see if you can find a 4 core cable with similar core size and extend those separately. 

It might work, if not you can still rejoin the cables without extending using a ‘chocolate block’ and taping it all up to get back to where you started.

I've ordered some of these. 

https://docs-emea.rs-online.com/webdocs/06e8/0900766b806e809c.pdf

They are rated to 10 amp so i'm hoping that's plenty. 

What about the cable, or are you just going to remake the join?

That caused me some problems too! Luckily I found some Trailer Cable that is 7 core and think (hope) that's enough!

The cores of the trailer cable are 1mm thick

I'm working on a 5 amp maximum. The plugs and sockets are rated to 10 amps.